Windblown Sandstones Cemented by Sulfate and Clay Minerals in Gale

PUBLICATIONS Geophysical Research Letters RESEARCH LETTER Wind-blown sandstones cemented by sulfate 10.1002/2013GL059097 and clay minerals in Gale Crater, Mars Key Points: R. E. Milliken1, R. C. Ewing2, W. W. Fischer3, and J. Hurowitz4 • Lower Mt. Sharp in Gale Crater exhibits evidence for wind-blown 1Department of Geological Sciences, Brown University, Providence, Rhode Island, USA, 2Department of Geology and sandstones 3 • Preserved dune topography is indicative Geophysics, Texas A&M University, College Station, Texas, USA, Division of Geological and Planetary Sciences, California 4 of specific environmental conditions Institute of Technology, Pasadena, California, USA, Department of Geosciences, Stony Brook University, Stony Brook, New • Some preserved dunes contain clays, York, USA possibly as authigenic cements Abstract Gale Crater contains Mount Sharp, a ~5 km thick stratigraphic record of Mars’ early environmental Supporting Information: • Figures SA1–SA8, Tables S1, and S2 history. The strata comprising Mount Sharp are believed to be sedimentary in origin, but the specific • Readme depositional environments recorded by the rocks remain speculative. We present orbital evidence for the occurrence of eolian sandstones within Gale Crater and the lower reaches of Mount Sharp, including Correspondence to: preservation of wind-blown sand dune topography in sedimentary strata—a phenomenon that is rare on Earth R. E. Milliken, [email protected] and typically associated with stabilization, rapid sedimentation, transgression, and submergence of the land surface. The preserved bedforms in Gale are associated with clay minerals and elsewhere accompanied by typical dune cross stratification marked by bounding surfaces whose lateral equivalents contain sulfate salts. Citation: Milliken, R. E., R. C. Ewing, W. W. Fischer, These observations extend the range of possible habitable environments that may be recorded within Gale and J. Hurowitz (2014), Wind-blown Crater and provide hypotheses that can be tested in situ by the Curiosity rover payload. sandstones cemented by sulfate and clay minerals in Gale Crater, Mars, Geophys. Res. Lett., 41, 1149–1154, doi:10.1002/2013GL059097. 1. Introduction Received 19 DEC 2013 The ~5 km tall mountain in the central region of Gale Crater, colloquially known as Mount Sharp, contains Accepted 30 JAN 2014 what may be the thickest exposure of sedimentary rock on Mars. However, the mechanisms responsible for Accepted article online 31 JAN 2014 Published online 22 FEB 2014 the formation of these rocks remain poorly understood and include lacustrine deposition, accumulation of volcanic ash, eolian deposition, ancient polar deposits, and precipitation from giant springs [Cabrol et al., 1999; Malin and Edgett, 2000; Milliken et al., 2010; Greely and Guest, 1987; Schultz and Lutz, 1988; Rossi et al., 2008; Thomson et al., 2011; Wray, 2013; Kite et al., 2013]. Each of these hypotheses carries different implications regarding former habitability and the likelihood for preservation of biosignatures [Summons et al., 2011]. Data collected from Curiosity will be important in testing these ideas, but because of the great size of Mount Sharp, orbital observations on the sedimentary processes recorded in its strata are key to expanding and contextualizing discoveries made in situ along the rover traverse path. Orbitally acquired Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) visible-near-infrared reflectance spectra of the strata in Mount Sharp exhibit absorptions in the 0.4–2.5 μm wavelength range that are consistent with the presence of primary igneous minerals (pyroxene and olivine) as well as Fe-bearing clay minerals, hydrated sulfate salts (likely Mg varieties), and red hematite that occur in specific horizons within the strata [Milliken et al., 2010]. Their detections are restricted to strata in the Lower formation of the mound, which is unconformably overlain by the more dust-covered Upper formation [Milliken et al., 2010]. The presence of hydrous minerals and fluvial geomorphic features [Milliken et al., 2010; Anderson and Bell, 2010; Thomson et al., 2011; Wray, 2013] suggest liquid water was once present in the crater, but the relative contributions of groundwater versus surface runoff are unknown. The duration and absolute age of this aqueous activity are also largely unknown, but crater counts place the formation of Gale Crater and the Lower formation of Mount Sharp along the Noachian-Hesperian time-stratigraphic boundary (3.5–3.7 Ga) [Thomson et al., 2011]. 2. Results Images acquired by the High Resolution Imaging Science Experiment (HiRISE) reveal that some strata within and onlapping the Gale Lower formation are likely sandstones. Strata exposed in cross section in the lowermost portion of the Lower formation exhibit horizontal or low-angle bounding surfaces that are laterally MILLIKEN ET AL. ©2014. American Geophysical Union. All Rights Reserved. 1149 Geophysical Research Letters 10.1002/2013GL059097 a) c) 25 km 50 m b) d) 50 m 50 m Figure 1. The ~155 km diameter Gale Crater on Mars, centered at 137.7°E, 5.3°S. (a) Mars Orbiter Laser Altimeter map overlain on a Context Imager mosaic. Black markers indicate locations of best examples of filled fractures, red indicates locations of preserved bedforms and yellow indicates preserved bedforms with clay signatures. Green star = Figure 1b; blue star = Figure 1c-1d; red triangle = Figure 2a; red square = Figure 2b; red star = Figures 2c–2e; yellow circle = Figure 4; yellow box = Figure SA3,SA4. (b) Dipping beds (white arrows) exhibiting downlap exposed in a canyon in western Mount Sharp. (c) Low-angle bedding observed in the Lower formation of Mount Sharp and (d) interpretation of cross-stratification and bounding surfaces (Figure 1d). Direction of North: (Figure 1a) up, (Figure 1b) left, and (Figures 1c and 1d) down. See Table SA1 for HiRISE image IDs. continuous over hundreds of meters and separated by tens of meters of inclined stratification (Figure 1 and Figure SA1 in the supporting information). This stratigraphic architecture is interpreted as ~20–40 m thick sets of dune cross stratification. Subcritical angles of bedform climb are documented by foresets truncated by an upper bounding surface and bottomsets that downlap onto a lower bounding surface. CRISM spectra for this a) c) d) d 100 m 100 m b) e) e 100 m 200 m 100 m Figure 2. Examples of preserved dune bedforms. (a) Bedforms in a unit along the southern margin of Mount Sharp; retention of craters and a cross-cutting fracture (northwest portion) indicate the unit is lithified. (b) Bedforms on an exposed surface in Mount Sharp near the Lower formation-Upper formation contact. (c) Example of preserved bedforms undergoing erosion and transitioning to washboard morphology. (d) Close-up of washboard that results from preferential preservation of interdune materials by erosion of (e) crests in preserved bedforms. White arrows indicate bifurcation. North is up in all images. See Table SA1 for HiRISE image IDs. MILLIKEN ET AL. ©2014. American Geophysical Union. All Rights Reserved. 1150 Geophysical Research Letters 10.1002/2013GL059097 a) location indicate the presence of a hydrated phase but are too noisy to allow more specificphase identifications; spectra of equivalent strata in other locations of the Lower formation indicate the presence of hydrated sulfates [Milliken et al., 2010]. Another example of this style of stratification can be observed in the wall of a large canyon in the western portion of the Lower formation (Figure 1b). Here we interpret a ~50–75 m thick unit to exhibit ~25 m thick subcritically climbing dune sets with foresets that have an apparent dip of approximately 30°— this crest-perpendicular or slightly oblique cross section can be used to estimate the true direction of dune migration at ~150° from North. At the same location, beds in overlying and underlying units do not exhibit such steep apparent dips and 250 m may represent either oblique cross sections or preservation of the lower, downlapping portion of b) the dune set or low-angle sand sheet cross stratification [Kocurek and Nielson, 1986]. The style of stratification shown in Figure 1 is consistent with an eolian origin and exhibits relatively regular set thicknesses, which is generated by regular crest spacing and trough elevation of migrating dunes. Though regular set thickness may arise with any type of dune 50 m accumulation—wet, damp, or dry—in many terrestrial environments where this is observed, Figure 3. Preserved bedforms overlain by fan deposits along interdune areas tend to be damp or wet (e.g., Entrada the SW margin of Mount Sharp. (a) Fan deposit derived from incision of the Lower formation showing inverted topography; Sandstone; rare areas of the Navajo Sandstone) (b) close-up of lighter-toned sediments associated with the fan [Kocurek, 1981]. In the case of wet or damp in Figure 3a that overlie darker preserved bedforms and are conditions, accumulation of eolian sediment is driven undergoing erosion. Note the bifurcation and preserved cra- by a relative rise in the groundwater table that ters in the bedforms. North is up in all images. HiRISE image promotes early diagenesis, cementation, and thus ESP025012_1745. limits the depth of scour down to the capillary fringe. Sandstones have been observed elsewhere on Mars by the Opportunity rover in Meridiani Planum, where ~80–100 m of section was interpreted as a dune-playa environment that experienced intermittent flooding by rising groundwater [Grotzinger

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